There is a continuing need for further refinement of air-fuel ratio (A/F) control in vehicular internal combustion engines. At present, A/F is managed by a powertrain control module (PCM) onboard the vehicle. The PCM is suitably programmed to operate in response to driver-initiated throttle and transmission gear lever position inputs and many sensors that supply important powertrain operating parameters. The PCM comprises a digital computer with appropriate processing memory and input-output devices and the like to manage engine fueling and ignition operations, automatic transmission shift operations and other vehicle functions. In the case of such engine operations, the computer receives signals from a number of sensors such as a crankshaft position sensor, and an exhaust oxygen sensor.
Under warmed-up engine operating conditions, the PCM works in a closed loop continuous feedback mode using the voltage signals from an oxygen sensor related to the oxygen content of the exhaust. The crankshaft angular position information from the crankshaft sensor and inputs from other sensors are used to manage timing and duration of fuel injector duty cycles. Zirconia-based, solid electrolyte oxygen sensors have been used for many years with PCMs for closed loop computer control of fuel injectors in applying gasoline to the cylinders of the engine in amounts near stoichiometric A/F. The PCM is programmed for engine operation near the stoichiometric A/F for the best performance of the three-way catalytic converter.
With more strict emission standards gradually phasing in, there is a need for further refinement of automotive technologies for emissions reduction. One such refinement is the use of a linear response (wide-range) A/F sensor in the exhaust pipe(s) in place of the current zirconia switching (nonlinear) oxygen sensor. Experiments have demonstrated that significant reductions in tailpipe NOx emissions are possible because of the more precise A/F control offered by a linear A/F sensor.
A second refinement is to increase vehicle fuel economy by diluting the air-fuel mixture with excess air (lean burn) or with exhaust gas recirculation (external EGR). The maximum benefit is achieved at the highest dilute limit. However, in a multi-cylinder engine, the limit is constrained by the occurrence of partial burns and possibility of misfire in the cylinder(s) containing the leanest mixture. This happens due to maldistribution of air, fuel or EGR in different cylinders. Thus, a new capability for the control of each individual cylinder air-fuel ratio by software is needed. Here, the intention would be to control only one variable (e.g., air, fuel or spark) to create uniform A/F or torque in all cylinders since only a single variable (e.g., A/F, O2 or torque) would be measured. Clearly, single-loop feedback controllers around various sensors can operate independently to control air, fuel or spark in every cylinder.
Another motivation for all-cylinder A/F control is cost containment. For very low emission applications, fuel injectors of high precision (i.e., very small tolerances of less than 3%) are thought to be required. Achievement of this degree of tolerance, if possible at all, would be costly. A better solution would be to have software means to compensate for the differences between fuel injectors in real-time operation of the engine. Another source of cylinder imbalances in a multi-cylinder engine is the inherent engine maldistribution due to variable breathing capacities into various cylinders. The air maldistribution can result in A/F or torque imbalances for which a software solution is sought.
A new emission reduction strategy was developed for detecting and correcting fuel, air or spark imbalances between cylinders of a three-cylinder gasoline engine. That process is disclosed in U.S. Pat. No. 6,668,812, titled “Individual Cylinder Controller for Three-Cylinder Engine” and assigned to the assignee of this invention. This process for a three-cylinder engine has to be modified and expanded for a four-cylinder engine and an individual cylinder control strategy now needs to be developed for a four-cylinder gasoline engine.